After nearly 33 months of EME activity and recent improvements made, I thought it was a good time to try to quantify my current station performance using the latest tools at my disposal.
Recently, some improvements were made on my TX and RX capability, which I believe takes my EME station to its best performance level so far. On the TX side, my PA is now capable of delivering a continuous 1200 watts in JT65, which adds a solid and precious dB to my TX output compared to past configuration.
On the RX side, I found and eliminated a faulty component in my RX line which had been degrading my RX performance for sometime, so my station is now back to what it is fully capable of delivering.
So how capable is my station RX-wise at this juncture and how can this be assessed? What is the smallest station I can expect to work?
In order to answer the fundamental questions above, as described in previous sections, the ideal tool to quantify my station performance is without a doubt the Moon Echo capability, which enables me to produce and analyze the strength of self-produced Moon Echo signals.
After carefully optimizing the polarity of the TX array (2X14RPOL) in order to yield the strongest Echo returns on my 2X13 RX array, a set of 31 echo measurements were made, and the data was recorded for statistical analysis. The picture and video to the right show the complete Echo experiment in question.
RX PERFORMANCE ASSESSMENT
As one can see in the video, outstanding Moon Echos were obtained throughout the experimental run. The echo signals ranged from -13.9dB to -26.5 dB, with an average of -20.4dB and a standard deviation of 3.4dB. For those who are less familiar with statistical concepts, standard deviation is a measure of the variation within a data set. Many Moon Echos were between -15dB and -19dB, which makes feel very good about my RX capability and my setup, especially considering that I live in an urban environment.
How can this data be utilized in order to quantify the RX performance of my EME station? Very simple. Since we know the average signal strength of the Echos produced (-20.4dB), and we know the TX source employed to produce the Echos in question (2x14 and 1200 watts = 96,500 watts EIRP), it is possible to apply basic regression and extrapolation principles in order to deduce the RX capability at its extreme limits.
For instance, assuming no Ground Gain for now, and considering that the TX source employed can produce an average echo signal of -20.4 dB, one could easily postulate that cutting the power or reducing the size of the TX source by half should decrease the RX signal by about 3 dB. For example, a station using a 1x14 (instead of a 2x14) and 1200 watts should produce an average signal of -23.4 dB (i.e. -20.4dB -3dB). Simple right?
***WARNING: For Best Video Resolution, it is critical to select 720p HD resolution in the youtube "settings" at bottom right of the player. The 360p default setting won't yield good enough resolution to see the details. It will take several seconds before the High Resolution kicks in, so you will need to restart the video from the beginning when the High Resolution is active and select to view the video in "Full Screen Mode" for best experience...***
If we keep applying the same logic and further reduce the TX station size by half again, we could postulate that a station using a 1x14 and 600 watts this time (instead of 1200 watts) should produce an average EME signal of -26.4dB (i.e. -23.4dB -3dB). It is clear that an average signal of -26.4dB is more than sufficient to yield a successful EME QSO. Obviously, this is assuming no loss due to Faraday Rotation and Non-Reciprocity. In my case, as demonstrated numerous times, the RPOL is fully capable of mitigating against all these potential issues.
If we keep pushing the envelop and reduce the TX even further, but this time by only 2dB instead, we could hypothesize that a station using a 1X10 and 600 watts should produce an average signal of -28.4dB (i.e. -26.4dB -2dB). As demonstrated in the Section 44, an average signal around -28dB is still sufficient to yield a successful QSO, although a little patience may be required.
Obviously, there is a large number of possible scenarios here. The table to the right summarizes the thought process in more details. The baseline is highlighted in purple, where an average EME signal of -20.4dB was obtained from a 2X14 and 1200 watts, which equals to 96,500 watts EIRP. The table contains a column with NO Ground Gain, and columns with 3dB and 4dB of Ground Gain. As the EIRP of the TX station decreases, we can see how the signal gets weaker. Since the EIRP figures do not easily tell us how these translate into in the real world, a column was added with examples of EME stations corresponding to these EIRP figures.
What signal strength threshold should be used? Based on the WSJT decode experiment described in Section 44, it is probably fair to assume that a minimum average signal of -29dB is needed in order to produce the necessary decodes to yield a successful QSO. Obviously, a threshold of -30dB could be considered as well, but the degree of difficulty becomes quite significant so a more conservative number (-29dB) is preferred.
As an example to show how this simple table works, one could say that an EME station of 15,000 watts EIRP and no Ground Gain should produce an average signal at my station of about -28.5dB, which is very close to the threshold limit established (-29dB). That EIRP level is equivalent to stations like 1X10/500, 1X12/400 or 2X7/400. Being so close to the RX threshold limit established, these are probably the smallest stations I can expect to work when no ground gain is involved. With 3dB of Ground Gain, the average RX signal would obviously be 3dB stronger, that is -25.5dB, and smaller stations could be worked.
Are these extrapolated RX performance figures close to reality? Based on my experience, they absolutely are and match pretty nicely with real world QSO's I have made. For instance, as shown in Section 39, I recently worked ZL3NW (Rod), who was running a 1X12 and 500 watts with good Ground Gain (let's assume 3dB of Ground Gain). We can see in the table that at 15,000 watts EIRP, which is a good approximation of a 1X12/500 station, the expected average signal with 3dB ground gain is around -25.5dB. As shown in the video of my QSO with Rod-ZL3NW, his actual average EME signal was -26.3dB, which is very close to the expected RX figure suggested in the table. Another video example can be seen in Section 9 where I decoded F4EZJ for over an hour who was only using a Single 8-element Yagi with 500 watts and with the help of some ground gain, was producing an average signal in the -26dB range, again very close to the 10,000 watts EIRP figures suggested in the table with 3-4dB of Ground Gain.Using the same validation method using average signal strength of stations I made QSO's with, the table above always provided pretty good match with the reality.
Obviously, one must remember that the figures calculated in the table are only valid for the conditions at which the initial Echo data was collected, i.e. at a degrade factor of -2.0dB. This does not really represent a problem as it is easy enough to make simple adjustments to the figures in the table based on different degrade factors. For instance, if the degrade factor is at -3dB (i.e. yielding EME signals 1dB weaker compared to a degrade of -2dB), all the figures in the table would be 1dB weaker. Very simple...
The evaluation technique describe above seems to provide excellent estimates of the RX performance that can be expected from an EME station and match nicely what I am seeing in the real world. I do have a lot of video evidences to back that up. As I keep improving my station, I will update these figures and try to further improve this technique.